It is known in the art to prepare polytetramethylene ether glycols by polymerizing tetrahydrofuran in the presence of a cationic initiator, generally of the Lewis acid type.
An article by Dreyfus and Dreyfus, "Advances in Polymer Science," 4 528 (1967) reviews the types of catalyst which have , , previously been used. They include:
(1) Metal halide adducts with active hydrogen containing compounds, e.g. FeCl.sub.3 or AlCl.sub.3 with .alpha.-chloro-dimethyl ether or benzyl chloride. PA1 (2) Unsaturated tertiary oxonium salts. In these salts, the anion is usually derived from a Lewis acid, e.g., BF.sub.4.sup.-, SbCl.sub.6.sup.-. PA1 (3) Other materials including complex inorganic acids such as HClO.sub.4, HBF.sub.4, HSO.sub.3 F, H.sub.2 SnCl.sub.6 ; acylium salts from Lewis acids and acylating agents; aluminum alkyls and haloalkyls such as AlEt.sub.3, AlEt.sub.2 Cl, AlEtCl.sub.2 with a cocatalyst such as water or epichlorohydrin.
Many of the catalysts used to the present have given only liquid polymers and conversions have varied widely.
Cationic initiators which have been important commercially include strong, soluble acids such as fuming sulfuric acid and fluorosulfonic acid. Although the poly(tetrahydrofuran) diols produced using the soluble acid initiators have a sufficiently narrow molecular weight distribution (M.sub.w /M.sub.n less than about 2-3) for many applications, these processes generate large amounts of acidic wastes that require costly treatment and disposal. Another disadvantage is that product consistency is difficult to achieve.
U.S. Pat. No. 4,189,566, to Mueller et al. (1980), discloses a process for the preparation of polybutylene glycol carboxylic acid diesters by polymerizing tetrahydrofuran, where the tetrahydrofuran is treated before polymerization with a strong mineral acid and is polymerized in the presence of one or more carboxylic acids and/or anhydrides.
U.S. Pat. No. 4,243,799 (1981), to Mueller et al. provides a description of a process for the preparation of polybutylene glycol carboxylic acid diesters by polymerizing tetrahydrofuran, wherein the tetrahydrofuran after removal of the catalyst used in the preparation of tetrahydrofuran, is treated, before polymerization, with a strong mineral acid, an organic sulfonic acid, silica gel and/or bleaching earth, the treating agent is removed from the tetrahydrofuran and the tetrahydrofuran is then polymerized in the presence of one or more carboxylic anhydrides and a polymerization catalyst. The improvement comprises using as the polymerization catalyst a bleaching earth containing less than 3% by weight of water, said catalyst being arranged in a fixed bed, and passing a mixture of pretreated tetrahydrofuran and carboxylic anhydride through said fixed bed. Acetic anhydride is used in some examples, but it is mixed with the tetrahydrofuran.
The use of a "bleaching earth" catalyst avoids many of the problems of the soluble acid catalysts (see, for example, U.S. Pat. Nos. 3,433,829 and 4,189,566 and 4,243,799 supra). The bleaching earths known in the art include naturally occurring aluminum hydrosilicates and aluminum/magnesium hydrosilicates of the montmorillonite type. The clays are normally activated by acid washing. A carboxylic acid anhydride is used as an activator, and the resulting polytetramethylene ether polymer has ester end groups. The ester end groups can be converted to hydroxyl end groups by base-catalyzed transesterification with an alcohol (see U.S. Pat. No. 4,230,892) or by catalytic hydrogenation.
In U.S. Pat. No. 4,728,722, to Mueller et al. (1988) there is disclosed a process for the batchwise preparation of polyoxybutylene polyoxyalkylene glycols by copolymerizing tetrahydrofuran with a 1,2-alkylene oxide in the presence of compounds containing reactive hydrogen, the polymerization being carried out over a bleaching earth catalyst or zeolite catalyst.
It is stated that the improvement comprises feeding the 1,2-alkylene oxide to the reaction mixture in such a manner that the concentration of the 1,2-alkylene oxide in the reaction mixture is kept below 2%.
In U.S. Pat. No. 5,208,385, to Kahn et al. (1993), there is described a process for producing tetrahydrofuran polymers having a narrow molecular weight distribution which comprises polymerizing tetrahydrofuran in the presence of a carboxylic acid anhydride and an effective amount of an amorphous silica-alumina catalyst, wherein the amorphous silica-alumina catalyst has an Al.sub.2 O.sub.3 content with the range of about 10 wt % to about 30 wt % and wherein the resulting tetrahydrofuran polymer has a number average molecular weight within the range of about 200 to about 5000, and a molecular weight distribution less than about 3.
A significant disadvantage of the bleaching earth catalysts is that the polymers produced have higher polydispersities (M.sub.w /M.sub.n) than desirable, typically 3-4 at molecular weights of about 400 to 3000. It is well-known in the art that the molecular weight distribution (MWD) of the poly(tetrahydrofuran) impacts the properties of the polyurethanes or polyesters made therefrom. In general, mechanical properties of finished products are superior when poly(tetrahydrofuran) having a relatively narrow molecular weight distribution is used (see U.S. Pat. No. 4,933,503, Col. 2).
There are two general approaches to obtaining poly(tetrahydrofuran) having a relatively narrow molecular weight distribution. In one approach, poly(tetrahydrofuran) having a broad MWD is prepared, and the product is post-treated either by distillation to separate low molecular weight oligomers, selective depolymerization (see, for example, U.S. Pat. No. 4,363,924), selective solvent extraction with water/alcohol/hydrocarbon systems (see U.S. Pat. No. 4,762,951), or a combination of these techniques (see U.S. Pat. No. 4,933,503). All of these post-polymerization techniques are expensive, labor-intensive, and time consuming. The object of the second general approach is to eliminate the need for posttreatment by preparing poly(tetrahydrofuran) having a narrow MWD. In one method, a low concentration of an alkylene oxide must be maintained throughout the tetrahydrofuran polymerization (U.S. Pat. No. 4,728,722 supra). In another method, the mole ratio of the reactants and reaction temperature must be carefully controlled (U.S. Pat. No. 4,510,333).
U.S. Pat. No. 4,303,782, to McHale et al. (1981), discloses a method of polymerizing cyclic ethers especially tetrahydrofuran to form high molecular weight polymers using a zeolitic polymerization catalyst. The zeolite catalyst preferably has a Constraint Index from 1 to 12 and a silica:alumina ratio of at least 12. The method described is directed toward making a high molecular weight solid polymer. In addition, the yields appear to be low, ranging from 4-13%.
In the art relating to polymerization of tetrahydrofuran there is a need for a catalyst with the advantages of bleaching earth catalysts, but which gives polymers having a narrower molecular weight distribution and which, therefore, overcomes the need for posttreatment. If yield of narrow range molecular weight poly(tetrahydrofuran) were also greatly improved with a new catalyst it would constitute a major contribution to the art. If such a catalyst reduced acidic wastes, it would be substantially more promising commercially.
In addition, a process which possessed all of these advantages and which could be operated in a continuous fashion, rather than batchwise, would represent a process with great commercial potential, the equivalent of which would not appear to be known or currently available.